This invention is an improvement over the method used in making glass-ceramics for semiconductor doping as described in U.S. Pat. Nos. 3,907,618, 3,962,000, 3,998,667 and 4,282,282. The disclosure of these patents is herein incorporated by reference. These patents describe a material and a method to make a glass-ceramic material by first melting a special glass composition at a high temperature, casting it into a round billet, cooling it to room temperature and then partially crystallizing the glass by passing the glass billet through a heat treatment cycle. The treated billets are then turned round and cut into thin wafers for use as planar diffusion sources for doping silicon wafers in semiconductor production. In addition, U.S. Pat. No. 5,145,540 describes a CIP process for sintering powders to make ceramic bodies. The method combines 40-90% oxides and 10-60% of a glass powder. The process of this invention differs in that billets are made of glass powders containing up to about 10% oxide additions.
In planar diffusion doping, a planar (i.e., flat) surface of a solid dopant host and a planar surface of a silicon wafer to be doped are positioned close and parallel to each other during the diffusion cycle. The ceramic wafer evolves B203 which deposits on the silicon wafer during the diffusion cycle. The B2O3 then reduces to boron which in turn diffuses into the silicon wafer to create a semiconductor device.
The above four US patents give many compositions from the RO—Al203-B203-SiO2 system were RO is one or more alkaline earth oxides (BaO), MgO, etc.). The compositions that can be melted, cast into billets, cooled to a relatively low temperature or to room temperature, and then passed through a heat treatment cycle to partially crystallize the glass in a controlled manner into what is known as a glass-ceramic. A major criteria for the glass-ceramic process is to select a composition that does not devitrify (uncontrolled crystallization) when it is cast into billets and is cooled. If any uncontrolled crystallization occurs, the final billet will contain relatively large crystals that can cause problems in semiconductor processing. These problems can include trapping impurities during cutting of the planar diffusion sources which evolve during use or can include breaking of the source during use because of stresses concentrating at the crystal. The larger the diameter of the billet to be cast, the more difficult it is to cool the glass billet without devitrication. It is therefore becoming very difficult to meet the demands of the semiconductor industry for large diameter planar diffusion source using the glass-ceramic process. These problems are overcome using the “fused glass powder” process of this invention.
It is possible to adjust the glass compositions so that large diameter glass billets can be cast and cooled to room temperature without devitrification. However, these more stable compositions do not produce planar diffusion sources that are rigid enough when the billets are heat treated and cut into planar diffusion sources for use in semiconductor doping. These planar diffusion sources quickly warp during use making them unusable. My “fused glass powder” process overcomes these problems.